How to expand a PowerShell array when passing it to a function

Question

I have two PowerShell functions, the first of which invokes the second. They both take N arguments, and one of them is defined to simply add a flag and invoke the other. Here are example definitions:

function inner
{
  foreach( $arg in $args )
    {
      # do some stuff
    }
}

function outer
{
  inner --flag $args
}

Usage would look something like this:

inner foo bar baz

or this

outer wibble wobble wubble

The goal is for the latter example to be equivalent to

inner --flag wibble wobble wubble

The Problem: As defined here, the latter actually results in two arguments being passed to inner: the first is "--flag", and the second is an array containing "wibble", "wobble", and "wubble". What I want is for inner to receive four arguments: the flag and the three original arguments.

So what I'm wondering is how to convince powershell to expand the $args array before passing it to inner, passing it as N elements rather than a single array. I believe you can do this in Ruby with the splatting operator (the * character), and I'm pretty sure PowerShell can do it, but I don't recall how.

Answer 1

There isn't a good solution to this problem in PowerShell Version 1. In Version 2 we added splatting (though for various reasons, we use @ instead of * for this purpose).

Here's what it looks like:

PS> function foo ($x,$y,$z) { "x:$x y:$y z:$z" }
PS> $a = 1,2,3
PS> foo $a # passed as single arg
x:1 2 3 y: z:
PS> foo @a # splatted
x:1 y:2 z:3

Answer 2

Well, there may be a better way, but see if this works:

inner --flag [string]::Join(" ", $args)

Answer 3

Building on @EBGreen's idea, and a related question I noticed in the sidebar, a possible solution is this:

function outer
{
    invoke-expression "inner --flag $($args -join ' ')"
}

Note: This example makes use of the Powershell 2.0 CTP's new -join operator.

However, I'd still like to find a better method, since this feels like a hack and is horrible security-wise.

Answer 4

If you want a quick ready-made solution, you can copy paste mine:

<#
    .SYNOPSIS
    Asks a question and waits for user's answer

    .EXAMPLE
    Usage with shortcuts and without ReturnValue
    Invoke-Question -Question "What would you like" -Answers "&Eggs", "&Toasts", "&Steak"

    Shows the quesiton and waits for input. Let's assume user input is 'S', the return value would be 2 (index of "&Steak")

    .EXAMPLE
    Usage without shortcuts and with ReturnValue
    Invoke-Question -Question "What would you like" -Answers "Eggs", "Toasts", "Steak" -ReturnValue

    Shows the quesiton and waits for input. The answers are prefixed with numbers 1, 2 and 3 as shortcuts.
    Let's assume user input is 2, the return value would be "Toasts" (prefixed numbers are "index + 1")

    .EXAMPLE
    Usage from pipeline with default value
    @("Eggs", "Toasts", "Steak") | Invoke-Question -Question "What would you like" -ReturnValue -Default 2

    Shows the quesiton and waits for input. The answers are taken from pipeline and prefixed with numbers 1, 2 and 3 as shortcuts.
    Steak is marked as default. If user simply continues without a choice, Steak is chosen for her.
    However, let's assume user input is 1, the return value would be "Eggs" (prefixed numbers are "index + 1")
#>
function Invoke-Question {
    [CmdletBinding()]
    param(
        # Main question text
        [Parameter(Mandatory = $true)]
        [string] $Question,

        # Question description, e.g. explanation or more information
        [Parameter(Mandatory = $false)]
        [string] $Description = "",

        # Default answer as index in the array, no answer is selected by default (value -1)
        [Parameter(Mandatory = $false)]
        [int] $Default = -1,

        # Set of answers, if the label is given with & sign, the prefixed letter is used as shortcut, e.g. "&Yes" -> Y,
        # otherwise the answer is prefixed with "index + 1" number as a shortcut
        [Parameter(Mandatory = $true, ValueFromPipeline = $true)]
        [string[]] $Answers,

        # If set, returns a value of selected answer, otherwise returns its index in the Answer array
        [switch] $ReturnValue
    )

    begin {
        # init choices
        $choices = New-Object Collections.ObjectModel.Collection[Management.Automation.Host.ChoiceDescription]
        $answerNumber = 1
        $rememberAnswers = @()
    }

    process {
        #init answers
        foreach ($answer in $answers) {
            $rememberAnswers += $answer
            if ($answer -notmatch "&") {
                # add number if shortcut not specified
                $answer = "&$answerNumber $answer"
            }

            $choices.Add((New-Object Management.Automation.Host.ChoiceDescription -ArgumentList $answer))
            $answerNumber++
        }
    }

    end {
        # ask question and return either value or index
        $index = $Host.UI.PromptForChoice($Question, $Description, $choices, $Default)
        if ($ReturnValue) {
            $rememberAnswers[$index]
        } else {
            $index
        }
    }
}